Well I believe that the issue here was that I reset the usb drive before using on pi again. Sounds like the right thing to do.

Firstly, I used EaseUS Partition Manager to delete the partitions, then create a new logical partition (fat32) and format.

That process gave the problem I experienced on both pi2's.

Yesterday, I did it differently. I still used EaseUS PM to delete the partitions, then created a new primary partition (shouldn't make a difference as pi is still booting from sd card, no?) - But then I wiped all data on the drive. After that, for good measure, I used win10 to do a full format on the usb drive. After that it worked as expected...

So maybe the problem was with the process I used to try and get the card as 'sterile' as I could to avoid any issues.

Anyway, 3 xbain pi are now all running nicely again and instead I have big headache with mysensors/mycontroller build instead! - Will it ever end?